Selenium cell



0. SASLAW SELENIUM CELL Oct 16, 1945.

Filed Sept. 8, 1943 INVENTOR. O7'7'0 snsmw ATTORNEY Patented Oct. 16, 1945 SELENIUM CELL Otto Saslaw, Lyndhurst, N. 3., asslgnor to Federal Telephone and Radio Corporation, New, York, N. 1., a corporation of Delaware Application September a, 1943, Serial No. 501,532

2 Claims. (01. 111-104) This invention relates to selenium rectiflers and particularly to selenium cells in which the selenium layer is covered with a cellulose base lacquer. This is a continuation in part of my copending application Ser. No. 449,529, filed July 2, 1942, for Selenium cell and lacquer therefor.

The general purpose of this invention is to improve the voltage and rectifying characteristics of selenium cells. It includes in general the application to the selenium surface of a cellulose base lacquer and more particularly to the inclusion in the lacquer of a certain amount of moisture.

As is well known, selenium elements are ordinarily composed of a coating of crystallized selenium on a base plate. A counter electrode is applied over the selenium surface, and according to theory a blocking layer is formed under the counter electrode, thereby enabling the element to have the rectifying property of being more conductive in one direction than in the other In accordance with the invention as disclosed in my copending application, Serial Number 449.529, above identified, I have succeeded in increasing the voltage which can be applied across the cell and reducing the current flowing in the direction opposite that of normal curent forward flow. I have done this by coating the metallic selenium surface, prior to application of the counter electrode, with a cellulose base lacquer, which upon drying leaves a film on the surface of the selenium. In accordance with my present invention I have discovered that under dry atmospheric conditions the lacquer should be mixed with moisture in order to obtain its greatest effectiveness. plied to it by the use of a spray operated by the ressure of moisture-containing air.

The invention will be better understood from the following detailed description and the accompanying drawing, of which Fig. 1 is a plan view of a selenium cell embodying the invention;

Fig. 2 is a central cross section of said cell; and

Fig. 3 is a sectional diagram of a spraying apparatus for spraying humidified lacquer 0n the selenium surface in accordance with this invention.

Figs. 1 and 2 show a well known type of selenium disk such as is used for a rectifier. It comprises a base plate I, ordinarily of steel, on which is placed a layer of selenium 2. The selenium may be applied in any suitable manner such as by melting selenium powder, spreading the molten selenium over the base plate and then allowing it to cool and harden. When selenium is applied in this manner it should then be annealed, which may be done in a well known manner by heating it for a period of time at a temperature somewhat below the melting point of selenium and appropriate to induce crystallization, thereby enabling the cell to operate properly as. a rectifier. A suitable way of carrying out this heat treatment is first to compress the selenium coated disk in a press and heat it for about a half hour at a temperature of around 120 0., then remove the pressure and raise the temperature to about 214 C. for some hours.

In accordance with my invention there is applied to the crystallized selenium surface a. layer of humidified lacquer 3 prepared as described below.

In order to use this device as a rectifier a counter electrode 4 should then be applied over the selenium layer 3. This may conveniently be done by spraying over the dried lacquer a coating of molten metallic alloy such as Wood's metal which when cooling leaves a thin metallic layer.

I have found that lacquers having a cellulose 'base are especially suitable for use in accordance with this invention. Such lacquers include those composed in large part of regenerated cellulose,

, cellulose esters or cellulose ethers and particu- This moisture may conveniently be aplarly those containing the lower alkyl radicals. These include cellulose formate acetate and propionate, and also methyl, ethyl and propyl cellulose. Cellulose nitrate has likewise been found to be especially effective as abase for lacquer used in the indicated manner.

The lacquers may be prepared in the usual manner, including a suitable volatile solvent which must be of 'a, type thatwill not have any effect On the selenium.

I have found that the incorporation of a suitable conductive constituent in the lacquer improves substantially the forward conductivity of the cellulose. Selenium dioxide and the selenium iodide have been found to be especially suitable for the purpose, and may be incorporated separately or in combination in suitable proportions, the conductivity being in general increased as the proportion of such constituent is increased.

The suitable lacquer formulas are as follows.

I Example 1 l Parts Selenium dioxide 3 Cellulose acetate lacquer. 4 Thinner All parts are by weight.

The cellulose acetate lacquer may be composed of four parts of jcellulose acetate dissolved in about 60 parts of methyl Cellosolve" acetate, known chemically I as ethylene glycol monopractical.

Example 2 Three components, A, B and C are prepared as follows: I

Component A is made by dissolving two parts of cellulose acetate in 60 parts of methyl Cellosolve acetate.

Constituent B i the thinner consisting of methyl Cellosolve acetate.

Constituent C is a 5% solution of the conducting material such as selenium dioxide in absolute ethyl alcohol. r

These constituents are mixed for use in proportions of one part of A. 5 parts of C, and a sufficient proportion of B to produce the desired consistency, usually between 2 and parts, 5 parts having been found suitable for preparing a spraying lacquer.

When cellulose nitrate is used it is employed in the same proportions as cellulose acetate. It has also been found that acetone is suitabe for use as a thinner instead of some or all of the methyl Cellosolve acetate.

As disclosed in my earlier application, Serial Number 449,529, the lacquer is' applied to the selenium surface to form the layer 3 in any convenient manner as by brushing or spraying. The volatilization of the thinner leaves a very thin layer of the cellulosic base mixed with the conductive material. It is desirable to make certain that the layer 3 is extremely thin, a factor which can readily be controlled by adjusting not only the amount of lacquer that is applied but also the viscosity of the lacquer. While various thinners and solvents may be employed, it is necessary that where the conductive material, such as selenium dioxide, is introduced by means of the thinner, such thinner must be a solvent of such material. Where the conductive substance is introduced by dissolution in a special solvent such as an alcohol, as in Example 2, the solvent must be compatible with the remaining ingredients and in particular must be capable of rapid and uniform dispersion throughout the cellulosic solution. Such solvent likewise must be free from harmful action on the selenium.

Attention is directed to the fact that while I have disclosed above, for the purpose of more fully completing the background of the subject matter of the present invention, the use of cellulose base lacquers, specific examples of which were derived from my earlier application, Serial Number 449,529, and have also indicated the general use' as commonly exists during winter months, the

atmospheric moisture may be insuflicient and accordingly moisture should be added artificially. A

suitable way of doing this is shown in Fig. 3, the

apparatus comprises a tank 6 containing some water 8 which does not fill the tank. The tank is covered by a suitable cover I to enclose the vapor. An air tube 9 is led into the tank through a suitable seal l0, passes up through the water and open into the air space above the water. Another tube i l is led from the air space in the tank through a seal l2 and opens into a spray chamber I3. The sprayer is of an ordinary type comprising a lacquer receptacle l4 containing. the liquid lacquer l5. Tube l6 dips into the lacquer and terminates at I! within the narrowed mouth i 8 of the sprayer. Compressed air is blown into the tank 6 through tube 9, and. mixes with the water vapor or steam in the tank above the water. To produce the proper proportion of water vapor the water may be heated as by a heating element l9, and may even be boiled if a lot of moisture is desired. The proper water temperature will depend somewhat upon the degree of dryness in the air at the time, and the provision of a substantial amount of moisture in the spray is especially necessary if the humidity of the surrounding atmosphere is below Under average conditions water temperatures ranging from 70 C. to 90 C. have been found to be satisfactory. The air mixed with moisture passes through tube II to the sprayer where it acts upon the spray device in the usual manner to force lacquer out with the lacquer is desirable for good results.

the mouth of the spray together with the moisture laden air, this mixes the moisture with the lacquer while it is bein applied to the selenium surface. If the moisture content of the air is sufficient the lacquer may be applied by brushing. It has been found that the moisture content should be equivalent to 50% humidity or greater; and if it is less than this amount, the air should be g'gp rly humidified to bring the humidity above After the application of the lacquer the thinner in it will volatize leaving a the base a very thin skin.- This enables the voltage applied across the element to be higher than otherwise, and reduces the current which can flow in the reverse direction. It is found that the current flowing in the forward direction is not reduced appreciably, however. Thus there is provided a selenium rectifier of high rectification ratio and high voltage breakdown quality.

While a preferred form of the invention has been disclosed together with certain modifications it will. be understood that other changes may be introduced within the scope of the invention as set forth in the claims.

What is claimed is:

1. The method of preparing a selenium cell which comprises applying to the surface of the selenium a cellulose lacquer containing moisture and an electrically conducting substance.

2. The method set forth in claim 1, in which the lacquer is applied to the selenium surface by spraying with moisture carrying gas under pressure.

OTIO SASLAW. 

